Wow, I just came across this problem.  I had no idea what was going on and why the PO was not coming up when I entered the bill.  When I started searching support it seems that this has been an issue for a few years now!  It never affected us because we usually have entered the bill before receiving the items so this is the first time it's happened.  Anyway it sounds like QuickBooks has not resolved this impending issue that's been an item of discussion for at least 2 years.